1. Field of the Invention
The present invention relates to an ink composition which is favorably used for inkjet recording; an inkjet recording method; and a printed material using the same. Further, the present invention relates to a planographic printing plate obtained by using the ink composition and a method of producing the planographic printing plate. Specifically, the present invention relates to an ink composition which is suitable for inkjet recording, which is curable with a high sensitivity to a radiation ray, and which is capable of forming a high quality image in inkjet recording and is superior in storage stability; an inkjet recording method; a printed material using the same; a planographic printing plate obtained by using the ink composition; and a method of producing the planographic printing plate.
2. Description of the Related Art
As image recording methods for forming an image on a recording medium such as paper based on image data signals, there are an electrophotographic method, a sublimation or fusion heat-transfer method, an inkjet method and the like. Among them, the inkjet method can be carried out using an inexpensive device, wherein the image is directly formed by ejecting an ink only on a necessary image region on the recording medium. Therefore, the ink can be efficiently used and the running cost is low. In addition, the inkjet method generates little noise and is superior as an image recording method.
The inkjet method allows printing not only on plain paper but also on a non-water absorptive recording medium such as a plastic sheet or a metal plate. However, higher speed and higher image quality at the time of printing are important issues, and the period needed for drying and hardening an ink droplet after ejection has a great influence on the printing efficiency and the quality of a printed image.
As one of such inkjet recording methods, there is a method using an inkjet recording ink which is curable by irradiation with a radiation ray. In this method, it is possible to improve the printing efficiency and form a sharp image, by hardening the ink droplet by irradiating it with a radiation ray immediately or after a given period from ejection.
If it is possible to raise the sensitivity of such an inkjet recording ink which is curable by irradiation with a radiation ray such as ultraviolet light and to thereby raise the efficiency of the ink hardening by the radiation ray, many benefits such as improvement in inkjet recording efficiency, reduction of power consumption, elongation of the lifetime of a radiation ray generator due to decrease in the load thereon, and prevention of evaporation of low-molecular substances due to insufficient hardening can be obtained. In addition, the improvement in sensitivity is effective in increasing the strength of the image formed with the inkjet recording ink, and in particular, when the ink composition is applied to preparation of planographic printing plates, it leads to an increase in the hardness of the image region and thus to an increase in printing durability.
Such an inkjet method using an ink composition which is curable by a radiation ray such as ultraviolet light has attracted attention recently, as the ink composition is relatively odorless and fast-drying and allows recording on a recording medium having less ink absorbency, and ultraviolet light curable ink compositions for inkjet, which are curable by radical polymerization, have been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-235382, 3-216379, and 5-214280, and Japanese Patent Application Publication (JP-B) Nos. 6-21256 and 6-62905). In addition, for the purpose of providing an inkjet recording ink that gives an image higher in sensitivity and adhesiveness to the recording medium without ink bleeding, even on a recording medium on which it is normally difficult to record directly by the inkjet recording method, and that is higher in stability with less skin irritation or sensitization, compositions containing polymerizable compounds of particular radically polymerizable acrylates and a colorant have been proposed (see, for example, JP-A Nos. 2003-192943 and 2003-192944). These radically polymerizable inks are superior in hardening speed and give an image without ink bleeding, but they have a disadvantage in that the adhesiveness to a recording medium deteriorates due to volume shrinkage during hardening.
Accordingly, for the purpose of improving the adhesiveness to a recording medium, cationically polymerizable ink compositions resistant to shrinkage during ultraviolet hardening have been proposed (see, for example, JP-A No. 9-183928). However, these cationically polymerizable inks have insufficient stability during storage because of the reaction of acids generated therein over time, which is a great obstacle for commercialization of these inks. For improvement in the storage stability, methods of adding a basic compound or a thermal base-generating agent have been proposed (see, for example, JP-A Nos. 2003-312121, 2003-341217 and 2004-91558), but these results in the emergence of a new problem in that the hardening efficiency of the ink is lowered by the basic compound, because it inhibits the function of the acid generated by light exposure. Thus, there exists a need for improvement in both storage stability and sensitivity.